1. Field of the Invention
The present invention relates to a reclining device attached to, for example, an automobile seat for appropriately adjusting inclination of a seat back with respect to a seat cushion and, in particular but not exclusively, to a method of making a component part of the reclining device.
2. Description of the Related Art
A reclining device is interposed between a seat cushion and a seat back that can be inclined relative to the seat cushion. A conventional reclining device generally includes a guide bracket fixed to one of the seat cushion and the seat back and an internal gear fixed to the other of the seat cushion and the seat back so as to be rotatable relative to the guide bracket.
The internal gear is first formed of a material having good press workability and then comes to have a predetermined surface hardness by a carburizing and hardening treatment and a subsequent tempering treatment in order to satisfy the strength requirements and the wear and abrasion resistance requirements.
The reclining device as disclosed in Japanese Laid-Open Patent Publication No. 10-276850 includes a first housing and a second housing opposed to each other. Each of the first and second housings has a plurality of projections integrally formed therewith on a side surface thereof, and each of a seat cushion frame and a seat back frame has a plurality of holes defined therein, into each of which one of the plurality of projections is inserted for subsequent welding thereof to the first or second housing, thereby joining the reclining device to the seat frame.
Japanese Laid-Open Patent Publication No. 11-20517 discloses another reclining device having a hardened layer that has been formed in an internal gear by induction hardening. The internal gear has a flange, on an inner peripheral surface of which internal teeth are formed, and the induction hardening is effected by induction heating the internal teeth from inside the flange using a high frequency and by subsequently cooling the internal teeth.
In the case of the reclining device as disclosed in Japanese Laid-Open Patent Publication No. 10-276850, because there is a possibility that a large load may be applied to the reclining device due to, for example, a vehicle collision, the plurality of projections formed with each housing must be made large, resulting in an increase in size of the reclining device. Further, a steel product that has undergone the carburizing and hardening treatment and the subsequent tempering treatment, i.e., the internal gear contains a considerable quantity of carbon in a surface layer thereof and, hence, it is most likely that weld cracks such as, for example, under-bead cracks may occur in the internal gear.
On the other hand, in the case of the reclining device as disclosed in Japanese Laid-Open Patent Publication No. 11-20517, when the internal teeth are heated from inside the flange, a connection between the flange and a portion of the internal gear positioned radially inwardly of the flange is also heated to thereby deform the flange, making it difficult to sufficiently ensure the accuracy after the hardening.